Lantibiotics are characterized by their post-translational modifications (PTMs). Dehydrations of serine and threonine residues into dehydroalanine and dehydrobutyrine residues, respectively, are common modifications found in lantibiotics. These dehydrated residues are cyclized with cysteines to form thioether bridges, which are called lanthionines1-2. Lantibiotics can contain other post-translational modifications, such as D-alanines in lacticin 3147, β-hydroxy aspartate in cinnamycin, 2-oxopropionyl in lactocin S, and an oxidized lanthionine in actagardine3-6.
The epidermin group of lantibiotics and other lantibiotic peptides have a S—[(Z)-2-aminovinyl]-D-cysteine (AviCys) residue at the C-terminal end of the peptide (FIG. 1a). This residue consists of a decarboxylated cysteine which forms a lanthionine ring. Several of these modifications are important for lantibiotic activity; however, the importance of the AviCys for the lantibiotic activity in the epidermin group of lantibiotics is not known.
Mutacin 1140, produced by Streptococcus mutans JH1140, is a lantibiotic that has shown promise as a potential therapeutic (FIG. 1b)7-9. It has a broad spectrum of activity against Gram-positive bacteria10. Further, mutacin 1140 has been shown to clear Staphylococcus aureus infections in rodent models with little toxicity11. The bacterium producing mutacin 1140 has been engineered into a therapy for preventing dental caries12.
Mutacin 1140 belongs to the class I epidermin group of lantibiotics and is structurally related to epidermin and gallidermin13-14. The first two lanthionine rings, rings A and B, of the epidermin group are referred to as the lipid II binding domain. The lantibiotic nisin shares structural homology to the lanthionine rings A&B. The latter half of the epidermin and nisin peptide is referred to as the lateral assembly domain, which presumably abducts lipid II into large lipid II/lantibiotic complexes15-16.
Decarboxylation of a C-terminal cysteine to form an AviCys residue occurs in several metabolites17. AviCys is present in the class II lantibiotics mersacidin and microbisporicin17-18. It is also found in non-lantibiotics, such as cypemycin. Cypemycin contains many of the lantibiotic PTM modifications; however, it does not form lanthionine rings19. The AviCys residue has also been found in the nonribosomal peptide synthetases (NRPS) produced metabolite thiovideramide. The mechanism of AviCys formation for the NRPS peptide maybe different due the nature of its biosynthesis20.
In certain lantibiotics, decarboxylation of cysteine at the C-terminus is performed by the flavoprotein LanD. This decarboxylase has been shown to be specific for C-terminal cysteines. Furthermore, LanD could not decarboxylate an ethyl-thioether mimic, suggesting decarboxylation occurs before ring D formation21. Crystal structures for both EpiD, the decarboxylase for epidermin, and MrsD, the decarboxylase for mersacidin, indicate that these enzymes form a homo-dodecamer22-23. Studies on the mechanism of activity suggest that decarboxylation produces an ene-thiol intermediate that promotes terminal ring formation24. There have been no reports of an isolated carboxylated analog of an AviCys containing lantibiotic, even in an EpiD deletion mutant of epidermin biosynthesis25.
HOAt/EDC coupling has been achieved for lantibiotics that contain a C-terminal carboxyl group. NVB302, an analog of actagardine which has undergone phase 1 clinical trials, has a diaminoheptane tail attached to the C-terminus of the lantibiotic27. Additionally, lantibiotics can be produced through solid-phase peptide synthesis using orthogonally protected lanthionine rings26, 28.